Image scanner using low-pixel-density CCD to achieve high-pixel-density scanning quality by pixel-level shift of CCD

ABSTRACT

An image scanner performing multiple scanning operations for a single scan line is disclosed. The additional scanning operations are performed between two adjacent pixels. For a double scanning mode, the CCD performs one scanning operation for a scan line at a first position. Then, the CCD is moved from the first position to a second position along the X-direction to perform the other scanning operation at the second position. The distance between the first and the second positions is a half of the distance between two adjacent pixels. The scanning data of the two scanning operations are alternatively arranged and converted into final image data. By this way, a low-pixel-density CCD can be used to obtain a high-pixel-density scanning quality.

FIELD OF THE INVENTION

The present invention relates to an image scanner which includes acharge coupled device (CCD) of a low pixel density, and particularly toan image scanner using the low-pixel-density CCD to achieve a betterscanning quality than it could. The better scanning quality can beobtained by performing a small-amount shift of the CCD.

BACKGROUND OF THE INVENTION

Due to the improvement on manufacturing techniques and the reduction ofcost, an image scanner becomes a popular peripheral equipment for apersonal computer. Meanwhile, owing to the enhancement of the processingspeed of the computer itself and the increasing requirement of users onthe scanning quality, the resolution of an image scanner is gettinghigher and higher. Therefore, it is a trend for the manufacturers ofimage scanners to improve the resolution as possible as they can.

The resolution of an image scanner is generally expressed by X DPI×Y DPIwherein DPI means Dot Per Inch. the number Y indicates the count ofscanning lines per inch, which is determined by the moving speed of acarriage driven by a stepping motor along the Y-axis. The number Xrepresents the optical resolution which is determined by the amplifyingrate of the lens device and the pixel number of the photoelectricconverting device such as a charge coupled device (CCD). For X=600 as anexample, it means that each inch of the scanned image in theX-direction, through the focusing and contraction of the lens device, isexposed by 600 pixels of the CCD.

In order to enhance the optical resolution of the image scanner, asoftware interpolation method or a CCD of a high pixel number can beoptionally used. If a software interpolation method is used to simulatea high-level state, no additional cost for hardware equipment is neededbut there is more or less distortion for the obtained image. If a CCD ofa high pixel number is used, the cost of the resulting image scannerwill multiply increase. Therefore, various researches are performed inorder to develop an image scanner of higher resolution, better scanningquality, and lower cost.

The Taiwanese Patent Publication No. 313286 filed Jul. 29, 1996, andincorporated herein for reference, discloses a twice-scanning process byusing a low-pixel-number CCD to scan a half of a document, and thenmoved along the X-direction to scan the other half of the document so asto double the resolution compared to the only once scanning operation bya CCD of the same pixel number. This process, however, still hasdisadvantages. First of all, the long distance movement of thelight-program device including the CCD and the lens device across a halfof the document along the X-direction has adversely effect onpositioning precision. In addition, the switching device for driving thelight-program device to move that long requires a complicated and bulkystructure, and thus consumes a lot of power. Further, it takes time tomove such a long distance.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide an imagescanner using a low-pixel-density CCD, which is moved in a pixel level,to achieve a high-pixel-density scanning quality in an efficient way.

A first aspect of the present invention relates to an image scanner forreading and converting an image of an object into digital image data.The image scanner includes a photoelectric sensing device including aplurality of sensing pixels arranged at equal spaces for converting anoptical signal from the object into an electric signal, and a shiftingdevice connected to the photoelectric sensing device for driving thephotoelectric sensing device from a first position to a second positionalong an X-direction to obtain a first electric signal and a secondelectric signal, respectively, wherein a distance between the first andthe second positions is less than a distance between two adjacentsensing pixels, and the first and the second electric signals arealternately arranged and converted to form the digital image data.

In an embodiment, the photoelectric sensing device includes a lensdevice for focusing and contracting the optical signal, a linear chargecoupled device consisting of the plurality of sensing pixels whichsenses the focused and contracted optical signal to generate theelectric signal, and a holder for mounting thereon the lens device andthe linear charge coupled device. The holder is driven by the shiftingdevice to allow the lens device and the linear charge coupled device tobe moved together from the first position to the second position.

Generally, the image scanner further includes a mirror set arrangedupstream of the lens device for transmitting the optical signal from theobject to the lens device, and a carriage housing mounted therein themirror set, the photoelectric sensing device and the shifting device,and moved along a Y-direction to scan the object.

In an embodiment, the shifting device includes a guiding track securedon the carriage housing for guiding the holder along the X-direction, afirst magnetic element secured on the holder, and a second magneticelement secured on the carriage housing and cooperating with the firstmagnetic element to generate a magnetic force. The magnetic forceincludes alternate attracting and repelling forces between the first andthe second magnetic elements so as to reciprocate the holder between thefirst and the second positions along the guiding track.

Preferably, the shifting device further includes two stoppers located bytwo opposite sides of the holder in order to further assure that theholder reciprocates between the first and the second positions.

Preferably, the first magnetic element is a first permanent magnet, andthe second magnetic element includes an electric motor and a secondpermanent magnet. The second permanent magnet is coupled to and drivenby the electric motor to rotate relative to the first permanent magnetto switch the magnetic force between the attracting and repelling forcesso as to reciprocate the holder.

Preferably, the distance between the first and the second positions isequal to a half of the distance between two adjacent sensing pixels.

A second aspect of the present invention relates to an image scannerwhich includes a photoelectric sensing device including a plurality ofsensing pixels arranged at equal spaces for converting an optical signalfrom the object into an electric signal, a carriage housing for mountingtherein the photoelectric sensing device, and a shifting deviceconnected to the carriage housing for driving the carriage housing tomove the photoelectric sensing device from a first position to a secondposition along an X-direction. The photoelectric sensing device isactuated at the first and the second positions to obtain a firstelectric signal and a second electric signal, respectively. A distancebetween the first and the second positions is less than a distancebetween two adjacent sensing pixels. The first and the second electricsignals are alternately arranged and converted to form the digital imagedata.

In an embodiment, the photoelectric sensing device includes a lensdevice for focusing and contracting the optical signal, a mirror setarranged upstream of the lens device for transmitting the optical signalfrom the object to the lens device, and a linear charge coupled deviceconsisting of the plurality of sensing pixels which senses the focusedand contracted optical signal to generate the electric signal.

Broadly, the term “image scanner” used herein includes any suitabledigital image pickup device which scanning an object and converting theimage of the object into digital image data. Further, the image scannercan be a reflective or a transmissive or a dual-mode image scanner.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may best be understood through the followingdescription with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram partially showing a preferred embodimentof an image scanner according to the present invention;

FIG. 2 illustratively shows a double scanning mode of the image scanneraccording to the present invention;

FIG. 3 schematically shows the elements included in the carriage of FIG.1;

FIGS. 4A˜4D schematically show four embodiments of the shifting deviceof FIG. 3, respectively, wherein a double scanning mode is performed foreach embodiment;

FIGS. 5A˜5C illustratively show three feasible ways to perform thedouble scanning mode of FIG. 4, respectively;

FIG. 6 is a schematic diagram partially showing another preferredembodiment of an image scanner according to the present invention; and

FIG. 7 schematically shows the elements included in the carriage of FIG.6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more specifically withreference to the following embodiments. It is to be noted that thefollowing descriptions of preferred embodiments of this invention arepresented herein for purpose of illustration and description only; it isnot intended to be exhaustive or to be limited to the precise formdisclosed.

Please refer to FIG. 1 which is a schematic diagram partially showing apreferred embodiment of an image scanner according to the presentinvention. The image scanner 20 includes a carriage 21 for accommodatingtherein an optical device 22 for picking up the image of a scannedobject (not shown). The carriage 21 is moved by a stepping motor 202along a pair of guiding shafts 201 in a Y-direction. For each step, theoptical device 22 is actuated to pick up a scan line of the image of anobject in an X-direction. With the movement of the carriage 21 along theY-direction, the entire object is scanned line by line.

The operational principle of the image scanner of FIG. 1 is illustratedas follows with reference to FIG. 2. For easy understanding, a doublescanning mode is given as an example. The term “double scanning”indicates that each scan line of the object is scanned twice, and thetwice scanning results are synthesized to obtain the scanned image data.

Differing from the above-mentioned conventional double scanning methodwhich scans the document by way of a left half scanning and a right halfscanning, the present double scanning method scans a document in aninterpolation manner. Please refer to FIG. 2. The optical deviceincludes a linear charge coupled device (CCD) which includes a pluralityof sensing pixels P equally spaced by a distance 2d, and is originallylocated at a first position 51. The sensing pixels P of the CCD pick upa scan line of image to obtain a first series of electric signals, andthe output is indicated by downward arrows A1. Afterwards, the CCD isshifted by a distance d along the X-direction to a second position 52 topick up the same scan line of image to obtain a second series ofelectric signals, and the output is indicated by upward arrows A2. Forsynthesis, the outputs A1 and A2 are alternately arranged as indicatedby the numeral reference 53, and then converted into image data. Asshown, the pixel density of the obtained image data according to thepresent invention is increased, thereby enhancing the resolution of theimage scanner.

Although a double scanning mode is given as an example, it is apparentfor those skilled in the art to apply the above principle to a multiplescanning mode by dividing the distance between two adjacent sensingpixels into equally spaced sections.

Please refer to FIG. 3 which schematically shows the elements includedin the carriage of FIG. 1. The carriage includes a carriage housing 211accommodated therein a mirror set 31, a photoelectric sensing device 32and a shifting device 33. The photoelectric sensing device 32 includes alens device 321, a CCD 322 and a holder 323. The lens device 321 and theCCD 322 are secured on the holder 323 so that the lens device 321 andthe CCD 322 will move together when the holder 323 is moved by theshifting device 33. The mirror set 31 is used for transmitting anoptical signal S1 from a scanned object 34 to the lens device 321. Thelens device 321 focuses and contracts the optical signal S1, and allowsthe focused and contracted optical signal S2 to be exposed oncorresponding sensing pixels of the CCD 322. The mirror set 31 includesone or more mirrors 311 depending on the desired light-transmissionroute between the scanned object 34 and the lens device 321. Theshifting device 33 includes a guiding track 331 allowing the holder 323to move therealong. The shifting device 33 further includes. drivingmeans 332 for moving the holder 323 along the guiding track 331according to a predetermined program.

In order to perform a double scanning mode as mentioned above, fourembodiments of the driving means 332 are described with reference toFIGS. 4A˜4D. It should be understood that the embodiments are given onlyfor illustration, and do not intend to limit the present invention.

Please refer to FIG. 4A. The driving means 332 includes a permanentmagnet 333 and an electromagnet 334. The permanent magnet 333 is securedon one side of the holder 32, and the electromagnet 334 is secured onthe carriage housing 211 (FIG. 3), and aligned with the permanent magnet333. A coil 335 is mounted around the electromagnet 334, and passestherethrough a current. By changing the direction of the current, e.g.clockwise or counterclockwise, the polarizing direction variesaccordingly. One polarizing direction makes the magnetic force betweenthe two magnets 333 and 334 become an attracting force, and the otherpolarizing direction makes the magnetic force become a repelling force.The switching between the attracting and the repelling forcesreciprocates the holder 32 along the guiding track 331 (FIG. 3). Inorder to make sure of the precise movement of the holder 32 between thefirst position and the second position (FIG. 2), two stoppers 40 arearranged by the opposite sides of the holder 32.

Please refer to FIG. 4B. The driving means 332 is similar to the oneshown in FIG. 4A except that an additional pair of magnets are providedto increase the magnet force.

Please refer to FIG. 4C. The driving means 332 includes a permanentmagnet 333, an electromagnet 334, a sustaining device 41 includes aspring element 411. The permanent magnet 333 is mounted in the holder 32and exposed to the air. The electromagnet 334 is secured on the carriagehousing 211 (FIG. 3), and aligned with the permanent magnet 333. Thespring element 411 is used to sustain against a bump 412 secured onanother side of the holder 32 when the holder 32 is shifted thereto. Acoil 335 is mounted around the electromagnet 334, and alternately passestherethrough a current. By conducting or interrupting the currentpassing around the electromagnet 334, the electromagnet 334 isintermittently polarized. The polarization of the electromagnetintroduces a repelling force between the two magnets 333 and 334, whichmoves the holder 32 toward the sustaining device 41 to compress thespring element 411. When the current is interrupted to retrieve themagnetic force, the holder 42 is pushed back by the restoring force ofthe spring element 411. Therefore, the holder 32 can reciprocatesbetween the first and the second positions. Likewise, two stoppers 40are preferably provided. The embodiment shown in FIG. 4C has anadvantage of less power consumption over the above two embodimentsbecause power is intermittently supplied to the shifting device.

Please refer to FIG. 4D. The driving means 332 includes a firstpermanent magnet 333, a second permanent magnet 336 and an electricmotor 337. The first permanent magnet 333 is mounted in the holder 32and exposed to the air. The second permanent magnet 336 is partiallyaligned with the first permanent magnet 333, and coupled to the electricmotor 337 which is mounted in the carriage housing 211 (FIG. 3). Thesecond permanent magnet 336 is driven by the electric motor 337 torotate relative to the first permanent magnet 333. Therefore, themagnetic force between the two magnets 333 and 336 is switched betweenan attracting and a repelling forces so as to reciprocate the holder 32between the first and the second positions (FIG. 2). Likewise, twostoppers 40 are preferably provided. The embodiment shown in FIG. 4D hasan advantage of even less power consumption over the above threeembodiments because no coil current is needed for generating a magneticforce.

As known from the above description, for a double scanning mode, theimage data are obtained by scanning twice for each scan line in theX-direction, and moving along the Y-direction to scan line by line. Inother words, the photosensing device 32 has to be moved along theX-directional guiding tracks 331 (FIG. 3), and also moved with thecarriage 21 along the Y-directional guiding shafts 201 (FIG. 1). Thereare several feasible ways to program the scanning path. It should beunderstood that the embodiments are given only for illustration, and donot intend to limit the present invention.

Please refer to FIG. 5A. First, the carriage is moved throughout thedocument along the Y-direction, as indicated by the arrow 61, whereinthe photoelectric sensing device is kept at the first position andperforms a scanning operation line by line. Then, the holder as well asthe photoelectric sensing device is moved to the second position alongthe X-direction, as indicated by the arrow 62. Subsequently, thephotoelectric sensing device performs another scanning operation line byline while moving back, as indicated by the arrow 63. The scanning dataare then synthesized to obtain the image data by proper hardware orsoftware.

Please refer to FIG. 5B. First, the carriage is moved throughout thedocument along the Y-direction, as indicated by the arrow 64, whereinthe photoelectric sensing device is kept at the first position andperforms a scanning operation line by line. Then, the carriage is movedback to the start position, as indicated by the arrow 65, and thephotoelectric sensing device is still kept at the first position. Next,the holder as well as the photoelectric sensing device is moved to thesecond position along the X-direction, as indicated by the arrow 66.Subsequently, the photoelectric sensing device performs another scanningoperation line by line while moving along the Y-direction again, asindicated by the arrow 67. The scanning data are then synthesized toobtain the image data by proper hardware or software.

Please refer to FIG. 5C. First, the photoelectric sensing deviceperforms a scanning operation at the first position for a first scanline. Then, the photoelectric sensing device is moved to the secondposition along the X-direction, as indicated by the arrow 691, toperform another scanning operation for the same scan line. Next, thecarriage is moved to a second scan line, as indicated by the arrow 68,and the photoelectric sensing device performs a scanning operation forthe second scan line at the second position. The holder is then movedalong the X-direction to the first position, as indicated by the arrow692, and the photoelectric sensing device performs another scanningoperation for the same scan line at the first position. Accordingly,each of the scan line is scanned twice, and the entire document isscanned. The scanning data are then synthesized to obtain the image databy proper hardware or software.

Alternatively, the movement of the photoelectric sensing device alongthe X-direction can also be achieved by moving the entire carriage. Thisembodiment will be described as follows with reference to FIGS. 6 and 7.The image scanner includes a carriage 61 for accommodating therein anoptical device 62 for picking up the image of a scanned object (notshown). The optical device 62 includes a lens device 621 for focusingand contracting an optical signal, a mirror set 622 arranged upstream ofthe lens device 621 for transmitting an optical signal from the objectto the lens device, and a linear CCD 623 consisting of a plurality ofsensing pixels to sense the focused and contracted optical signal togenerate an electric signal. All the above elements are secured on acarriage housing 611. The carriage 61 is moved by a shifting device 63in an X-direction, and moved by a stepping motor 602 along a pair ofguiding shafts 601 in a Y-direction. For each scan line, the opticaldevice 62 is moved with the carriage 61 along the X-direction andactuated multiple times to perform scanning operations. With themovement of the carriage 61 along the Y-direction, the entire object isscanned line by line. In order to move the carriage in sections alongthe X-direction, the shifting device 63 may be any suitable device ableto drive the carriage step by step in a pixel or sub-pixel level. For adouble scanning mode, any of the magnetic elements as shown in FIGS.4A˜4D can be used or modified to function as the shifting device.

From the above description, it is apparent that the moving distance ofthe lens and the CCD according to the present invention is much shorterthan that according to the Taiwanese Patent Publication No. 313286.Therefore, the unsatisfactory precision, complicated structure andpower-consumption problems encountered by the conventional technique canbe overcome by the present invention. Furthermore, compared to thesoftware interpolation method, the present invention is more veritable.

While the invention has been described in terms of what are presentlyconsidered to be the most practical and preferred embodiments, it is tobe understood that the invention need not be limited to the disclosedembodiment. On the contrary, it is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims which are to be accorded with the broadestinterpretation so as to encompass all such modifications and similarstructures.

1. An image scanner for reading and converting an image of an object into digital image data, the image scanner comprising: a photoelectric sensing device including a plurality of sensing pixels arranged at equal spaces for converting an optical signal from said the object into an electric signal; a shifting device connected to said the photoelectric sensing device for driving said the photoelectric sensing device from a first position to a second position along an X-direction to obtain a first electric signal and a second electric signal, respectively, wherein a distance between said the first and said the second positions is less than a distance between two adjacent sensing pixels, and said the first and said the second electric signals are alternately arranged and converted to form said the digital image data; a mirror set arranged upstream of said the lens device for transmitting said the optical signal from said the object to said the lens device; and a carriage housing mounting therein said carrying the mirror set, said the photoelectric sensing device and said the shifting device, and moved wherein the carriage housing is configured to move along a Y-direction to scan said the object.
 2. The image scanner according to claim 1 wherein said the photoelectric sensing device includes: a lens device for focusing and contracting said the optical signal; a linear charge coupled device consisting of said comprising the plurality of sensing pixels which senses said that sense the focused and contracted optical signal to generate said the electric signal; and a holder for mounting thereon said the lens device and said the linear charge coupled device, said the holder being driven by said the shifting device to allow said the lens device and said the linear charge coupled device to be moved together from said the first position to said the second position.
 3. The image scanner according to claim 1 wherein said the shifting device includes: a guiding track secured on said the carriage housing for guiding said the holder along said the X-direction; a first magnetic element secured on said the holder; and a second magnetic element secured on said the carriage housing, and cooperating with said the first magnetic element to generate a magnetic force which reciprocates said the holder between said the first and said the second positions along said the guiding track.
 4. The image scanner according to claim 3 wherein said the shifting device further includes two stoppers located by on two opposite sides of said the holder in order to further assure that said assist the holder reciprocates in reciprocating between said the first and said the second positions.
 5. The image scanner according to claim 3 wherein said the magnetic force includes alternate attracting and repelling forces between said the first and said the second magnetic elements.
 6. The image scanner according to claim 5 wherein said the first magnetic element is a first permanent magnet, and said the second magnetic element includes: an electric motor; and a second permanent magnet coupled to and driven by said the electric motor to rotate relative to said the first permanent magnet to switch said the magnetic force between said the attracting and repelling forces so as to reciprocate said the holder.
 7. The image scanner according to claim 5 wherein said the first magnetic element is a permanent magnet, said the second magnetic element is an electromagnet aligned with said the permanent magnet, and said the magnetic force is switched between said the attracting and repelling forces by changing a current direction passing around said the electromagnet so as to reciprocate said the holder.
 8. The image scanner according to claim 5 wherein said the first magnetic element includes two permanent magnets mounted on two opposite sides of said the holder, respectively, said the second magnetic element includes two electromagnets located by said the two opposite sides of said the holder and aligned with said the two permanent magnets, respectively, and said the magnetic force is switched between said the attracting and repelling forces by changing current directions passing around said the electromagnets so as to reciprocate said the holder.
 9. The image scanner according to claim 5 wherein said the first magnetic element is a permanent magnet, said the second magnetic element is an electromagnet aligned with said the permanent magnet, and said the magnetic force is switched between said the attracting and repelling forces by controlling whether a current passes around said the electromagnet so as to reciprocate said the holder.
 10. The image scanner according to claim 1 wherein said the distance between said the first and said the second positions is equal to a half of said the distance between two adjacent sensing pixels.
 11. An image scanner for reading and converting an image of an object into digital image data, the image scanner comprising: a photoelectric sensing device including a plurality of sensing pixels arranged at equal spaces for converting an optical signal from said the object into an electric signal; a carriage housing for mounting therein said carrying the photoelectric sensing device; and a shifting device connected to said the carriage housing for driving said the carriage housing to move said the photoelectric sensing device from a first position to a second position along an X-direction; wherein said the shifting device further includes two stoppers located by two opposite sides of said the photoelectric sensing device in order to further assure that said the photoelectric sensing device reciprocates between said the first and said the second positions; and wherein said the photoelectric sensing device is actuated at said the first and said the second positions to obtain a first electric signal and a second electric signal, respectively, a distance between said the first and said the second positions is less than a distance between two adjacent sensing pixels, and said the first and said the second electric signals are alternately arranged and converted to form said the digital image data.
 12. The image scanner according to claim 11 wherein said the photoelectric sensing device includes: a lens device for focusing and contracting said the optical signal; a mirror set arranged upstream of said the lens device for transmitting said the optical signal from said the object to said the lens device; and a linear charge coupled device consisting of said comprising the plurality of sensing pixels which senses said that sense the focused and contracted optical signal to generate said the electric signal.
 13. The image scanner according to claim 11 wherein said the carriage housing is further moved configured to move along a Y-direction to scan said the object.
 14. The image scanner according to claim 11 wherein said the distance between said the first and said the second positions is equal to a half of said the distance between two adjacent sensing pixels.
 15. A method of obtaining image data from an image scanner, comprising: transmitting an optical signal from a scanned object to a lens device with a mirror set wherein the mirror set is coupled to a carriage housing; transmitting the optical signal from the lens device to a photoelectric sensing device coupled to the carriage housing; obtaining a first signal from the photoelectric sensing device, the photoelectric sensing device comprising a plurality of sensing pixels arranged with approximately equal spacing with respect to each other; moving the photoelectric sensing device from a first position to a second position, wherein the distance between the first position and the second position is less than the distance between two adjacent sensing pixels; and obtaining a second signal from the photoelectric sensing device.
 16. The method of claim 15, further comprising: generating a magnetic force in a magnetic element disposed on the carriage housing to cooperate with a magnet coupled to the photoelectric sensing device to move the photoelectric sensing device from the first position to the second position along a guiding track disposed on the carriage housing.
 17. The method of claim 16, wherein the magnetic force includes alternate attracting and repelling forces.
 18. The method of claim 15, wherein the distance between the first and the second positions is equal to half of the distance between two adjacent sensing pixels.
 19. The method of claim 15, wherein the shifting device further includes two stoppers located by two opposite sides of the photoelectric sensing device to enable the photoelectric sensing device to move between the first and the second positions.
 20. An image scanner, comprising: a photoelectric sensing device having a plurality of sensing pixels configured to convert an optical signal into an image signal, wherein at least a portion of the plurality of sensing pixels are arranged with approximately equal spacing; a carriage housing for mounting the photoelectric sensing device; a shifting device coupled to the carriage housing and configured to move the photoelectric sensing device between a first position and a second position along an X-direction; wherein the shifting device further includes two stoppers located on two opposite sides of the photoelectric sensing device to assist the photoelectric sensing device in moving between the first and the second positions; and wherein the photoelectric sensing device is adapted to be moved between the first and the second positions to obtain a first image signal and a second image signal, respectively, wherein a distance between the first and the second positions is less than a distance between two adjacent sensing pixels.
 21. The image scanner of claim 20, further comprising: a lens device coupled to the carriage housing; and a mirror set coupled to the carriage housing.
 22. The image scanner of claim 20, wherein the carriage housing is configured to move along a Y-direction to scan an obiect.
 23. The image scanner of claim 20, wherein the distance between the first and the second positions is equal to a half of the distance between two adjacent sensing pixels.
 24. The image scanner of claim 20, further comprising: a guiding track disposed on the carriage housing for guiding the photoelectric sensing device along the X-direction; a first magnetic element coupled to the photoelectric sensing device; and a second magnetic element coupled to the carriage housing and cooperating with the first magnetic element to generate a magnetic force which moves the holder between the first and the second positions along the guiding track.
 25. The image scanner of claim 24, wherein the magnetic force includes alternate attracting and repelling forces between the first and the second magnetic elements.
 26. The image scanner of claim 24, wherein one of the first and second magnetic elements comprises a permanent magnet. 